Alcohol-aqueous phosphate buffer— Dissolve 0.29g of dibasic sodium phosphate in 450mLof water,add 550mLof alcohol,and mix.

0.05M Phosphate buffer— In a 1-liter volumetric flask,dissolve 6.9g of monobasic sodium phosphate in 500mLof water,dilute with water to volume,and mix.

Mobile phase— Prepare a suitable degassed solution of methanol and 0.05M Phosphate buffer(52:48)which,at ambient temperature and at a flow rate of 1.5mLper minute,gives a retention time of about 8.5minutes for dexamethasone phosphate.

Standard preparation— Using an accurately weighed quantity of USP Dexamethasone Phosphate RS,prepare a solution in Alcohol-aqueous phosphate bufferhaving a known concentration of about 30µg per mL.Prepare this solution fresh.

Assay preparation— Transfer an accurately weighed quantity of Cream,equivalent to about 3mg of dexamethasone phosphate,to a 150-mLbeaker.Add 65mLof Alcohol-aqueous phosphate buffer,and heat just to boiling.Pour the contents of the beaker into a 125-mLseparator containing 45mLof isooctane.After shaking for 1minute,decant the lower layer into a 100-mLvolumetric flask with the aid of a glass funnel.Rinse the 150-mLbeaker with two 15-mLportions of Alcohol-aqueous phosphate buffer,extracting the remaining isooctane in the separator with each portion and decanting the lower layer from each extraction into the 100-mLvolumetric flask.Dilute with Alcohol-aqueous phosphate bufferto volume,and mix.Filter through a membrane filter before injecting.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 254-nm detector and a 4-mm ×30-cm column that contains packing L1.Chromatograph five replicate injections of the Standard preparation,and record the peak responses as directed under Procedure:the relative standard deviation is not more than 1.5%.

Procedure— By means of a suitable sampling valve,separately inject equal volumes (about 20µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of C22H30FO8Pin the portion of Cream taken by the formula: in which Cis the concentration,in µg per mL,of USP Dexamethasone Phosphate RSin the Standard preparation,and rUand rSare the peak responses at equivalent retention times obtained from the Assay preparationand the Standard preparation,respectively.